Newsletter and Technical Publications
<International Source Book On Environmentally Sound Technologies
for Wastewater and Stormwater Management>

2.1 Impact of wastewater and stormwater

The impact of organic substances in wastewater is discussed in Section 1,
while the impact of pathogens and toxic chemicals in wastewater on human health
is discussed in detail in Appendix 2. The consequent loss of economic
productivity due to illnesses, medial bills or inability to perform work at full
capacity has been estimated to be in the order of billions of dollars per day.

Solids in both wastewater and stormwater form sediments and can eventually
clog drains, streams and rivers. Grease particles form scum and are
aesthetically undesirable.

The nutrients N and P cause eutrophication of water bodies, with lakes and
slow moving waters affected to a greater degree than faster flowing waters. In
the former the algae which are fertilised by the nutrients, settle as sediment
when they decay. The sediment acts as a store of nutrients and regularly
releases the nutrients to the water column, thus the cycle of bloom and decay of
the algae is intensified. In the early stages of eutrophication aquatic life is
made more abundant, because fish, for example, graze on the algae. With too high
a concentration of algae, the decaying algae contribute to BOD and the water is
deoxygenated. Thus wastewater, which has been treated to reduce BOD but still
high in nutrients, can still have a significant impact on the receiving water.
Some algae produce toxins which can be harmful to bird life and irritate skins
coming into contact with the water. Eutrophic water adds to the cost of water
treatment, when the water is used for drinking purposes.

Other pollutants in wastewater and stormwater are heavy metals and possible
toxic and household hazardous substances. Heavy metals include copper, zinc,
cadmium, nickel, chromium and lead. The content and concentration are dependent
on the pipe materials employed to convey drinking water, household cleaning
agents used, and for stormwater the type of materials used for roofing and
guttering. In high enough concentrations these heavy metals are toxic to
bacteria, plants and animals, and to people. Toxic materials may also be
disposed with household wastewater. These could be medicines, pesticides and
herbicides which are no longer used, excess solvents, paints and other household
chemicals. These substances can corrode sewer pipes and seriously affect
operation of treatment plants. They will also limit the potential of water
reuse, and therefore should not be disposed with household wastewater.

Spills of chemicals, particulates from motor vehicle exhaust and deposition
of atmospheric pollutants can similarly contaminate stormwater. These pollutants
will affect downstream receiving waters, and treatment systems if the stormwater
is treated.

Wastewater and contaminated stormwater can contaminate groundwater. This is
through infiltration of the wastewater or stormwater through the soil to
unconfined groundwater aquifer. Soil can filter some pollutants (see 2.2 Natural
purification processes), but soluble pollutants (e.g. nutrients and heavy
metals) and very small particles (e.g. virus) travel with the water to the
groundwater aquifer.

Heavy storm events can cause flooding. The effects of flooding can be severe.
Water levels in drain, stream and rivers rise considerably and the flow of water
can erode soils and embankments. Sediments which have been deposited in
quiescent stretches of a stream can be resuspended and transported further
downstream. In urban areas the water picks up litter and solid wastes in its
path as well as other diffuse pollution sources, and spread these in the
downstream flooded areas. Aquatic environments and water-fowl habitats can be
destroyed, and these may take some time to recover. The amenity value of these,
as well as recreational lakes, is therefore degraded. Engineered structures,
such as culvert and bridges, can be choked with wastes and debris, causing more
wide-spread flooded areas.